1. Field of the Invention
The present invention is directed to oligomers and in particular, to a class of relatively low-molecular weight oligomers which contain at least one isoimide group along with functional groups that enable the oligomers to be cured by addition polymerization. The invention is also directed to an improved process for producing such isoimide-containing oligomers essentially free of undesirable side reaction products.
2. Description of the Prior Art
Polymers formed of polyimides are well known in the art. One form of prior art polyimide polymer is described in U.S. Pat. No. 3,179,634, in which the polyimide polymers are characterized by repeating units having the formula: ##STR1## where R is a tetravalent aromatic group and R' is a divalent aryl group. Such polymers are known to be useful in the molding of films, fibers and the like having high tensile strength, good electrical properties and stability with respect to heat and water.
One of the primary drawbacks to polyimide polymers of the type described above arises from the method by which such polymers are prepared. Generally, the polymers are prepared by a condensation reaction in which a diamine is reacted with a dianhydride to form a corresponding polyamic acid. The polyamic acid can then be converted to the corresponding polyimide by heating. As those skilled in the art will appreciate, that condensation reaction is accompanied by the release of water vapor which can cause voids and other defects in molded products when liberated during a molding operation.
Substantial improvements over polyimide polymers of the type described in the above patent have been achieved as described in U.S. Pat. Nos. 3,864,309, 3,845,018, 3,879,349, and 3,928,450, the disclosures of which are incorporated herein by reference. As described in the foregoing patents, it has been discovered that the disadvantages of polyimide oligomers of the type described, and specifically the tendency of the liberated water vapor or other gaseous by-products to cause voids during molding, can be completely eliminated or substantially reduced when a polyimide oligomer is terminated with a terminal reactive group, such as an acetylenic group, capable of undergoing an addition polymerization reaction. As is described in the foregoing patents, acetylenic-terminated polyimide oligomers and other saturated polyimide oligomers can be polymerized, either with or without a catalyst, to form polymers which exhibit an extremely low void content along with high thermal stability characteristics and high structural strength.
While the polyimide oligomers containing acetylene groups described in the foregoing patents represent a significant advance in the art, they tend to have poor solubility in common lacquer solvents along with relatively high melting points, and their processibility to cured polyimides is difficult. Thus, at the high processing temperatures normally required, such polyimide oligomers cure too rapidly for use in many applications, particularly where the polymer is used to form a laminate over a large surface area.
U.S. Pat. No. 3,345,342 to Angelo, U.S. Pat. No. 3,261,811 to Tatum and U.S. Pat. No. 3,413,267 to Kruez disclose polyisoimides which can be used to form a polyimide by curing. Polyisoimides are prepared by reacting a carboxylic acid dianhydride with an aromatic diamine to produce a polyamic acid, and treating the resulting polyamic acid with a dehydrating agent. The polyisoimides of these patents are not disclosed as being oligomers but rather isoimide polymers and do not contain functional reactive end groups.
U.S. Pat. No. 4,097,456 to Barie and U.S. Pat. No. 4,133,792 to Antonoplos et al disclose improved processes for producing polyimide oligomers containing acetylene groups by carrying out the reaction in the presence of an N-methyl pyrrolidone solvent.